Online thinning for HybridGibbs

[chaozg]

General description of task

Add online thinning to HybridGibbs (online thinning for all other samplers was tracked in issue #677 )

Definition of Done (Feature/change)

• add support of thinning factor Nt to warmup and sample of HybridGibbs.
• add unit test

Definition of Done (Mandatory)

• Documentation added (docstrings on all public methods/classes)
• Unit tests added/updated (and passing!)
• Code reviewed and approved by 2 CUQI-maintainers
• Online documentation is rendered correctly (esp. math)

Definition of Done (Optional)

• [ NA] HowTo/tutorial added